The objectives are to test the following hypotheses: 1) Citrate transport across the brush border and the basolateral membranes of the renal epithelium is carrier-mediated. 2) In the brush border membranes, there is secondary active transport of citrate, energized by downhill Na ion transport. The carrier is shared with other endogenous organic anions. 3) In the basolateral membranes, the citrate carrier is shaped with other endogenous organic anions and with exogenous organic anions, like p-aminohippurate. Exchanges of organic anions across these membranes are likely to occur. To test these hypotheses, we will isolate and separate the two types of membranes by free-flow electrophoresis. Citrate transport across the vesiculated membranes may be passive by facilitated diffusion, primary active, or secondary active. To define this, we will study citrate transport under controlled conditions of temperature pH, and ionic concentrations. Conditions for each membrane must mimic the in vivo situation. For brush border membrane vesicles, a main condition is an inwardly directed Na ion gradient. For basolateral membrane vesicles, conditions include outwardly directed gradients for Na ion and organic anions other than citrate, and inwardly directed proton gradients.